This invention relates to manual resuscitators, and specifically to a manual resuscitator with control on tidal volume.
When spontaneous breathing has halted, the easiest method to reestablish breathing is through the application of mouth to mouth resuscitation. This is not always the most desirable method to use, and consequently a number of devices have been manufactured to manually ventilate a patient. The devices, commonly called manual resuscitators, consist essentially of an inflatable bug or reservoir, a face-mask or endotracheal tube connector, and connections between the reservoir and the mask. The resuscitators are either open to the air or attached to an oxygen enriched supply. The face mask is placed over the patient's nose and mouth, the reservoir is squeezed and gas is forced down the tube and into the patient's lungs. Once the operator stops squeezing the reservoir and releases the pressure, the patient exhales automatically and the reservoir assumes its original shape, drawing in gas for the next inflation. The process is then repeated. The resuscitators are adapted so that if the patient begins to breathe spontaneously, the resuscitator will not hinder this function.
The capacity of existing adult resuscitator reservoirs is typically between 1500 to 2000 cc, paediatric reservoirs from 600-750 cc, and infant reservoirs around 200 cc. Existing resuscitators generally are used and constructed in such a manner that the only indicators as to how much gas is being forced out of the reservoir and into the patient's lungs are the degree to which the patient's chest rises when the reservoir is squeezed, and the resultant back pressure felt by the squeezing hand.
The formula for calculating the average respiratory tidal volume is 10 ml of gas for every 1 kg of body weight. Using this formula, 700 ml would be the approximate tidal volume required for a 70 kg adult. Adult lungs are generally strong and resilient and consequently if too much gas is forced into the lungs few side effects will be observed. The lungs of a generally healthy 70 kg adult likely would not be seriously injured if a tidal volume as high as 2000 ml was used.
There is an acute problem in the case of infants, however, especially premature infants. Premature infants not only have less lung development than full term infants, and smaller tidal volumes, but also are far more likely to require resuscitation. Using the above formula, a 1 kg infant should have a tidal volume of about 10 ml, and in practice, about 10 to 16 ml is typical for a premature infant weighing about 1 kg. If the only mechanism for controlling the amount of gas delivered is squeezing the reservoir to varying degrees, it is extremely difficult for a clinician to administer 10-16 ml of gas from a reservoir with a capacity of 200 cc. Administering too much gas to the infant could very possibly rupture the lungs, causing the condition known as pneumothorax. This condition is frequently observed in infants who have been resuscitated using a manual resuscitator and then paced on an automatic ventilator. (The automatic ventilator has been blamed for this condition, but an increasing number of clinicians believe the condition may be caused by over-inflation of the lungs during manual resuscitation.)
Control of tidal volume is generally desireable in resuscitation, but is particularly desireable in the case of infant resuscitation. There are extremely few infant resuscitators on the market, and generally there are no controls of tidal volume. There are many adult resuscitators on the market, but generally they too have no controls of tidal volume.
It should be understood that although this invention is intended primarily for manual resuscitators for infants, it could be applied just as easily to manual resuscitators for adults, although there the need is not as great.